Born in 1460, Oura Mitsunobu was stationed at Tanesato Castle on the western coast of Tsugaru in 1491 by order of Nanbu Nobutoki, head of the Sannohe Nanbu clan, to guard against the Ando clan. In 1502 he built Oura Castle at the foot of Mount Iwaki and installed his adopted son Oura Morinobu as its lord, while he himself continued to be based at Tanesato, where he died in 1526 (aged 67, though some sources say 51). Mitsunobu was himself merely a local vassal of the Nanbu, but because his descendant five generations later, Oura (Tsugaru) Tamenobu, broke away from the Nanbu to found the independent Tsugaru clan, later generations came to regard Mitsunobu as the founding ancestor of the Tsugaru line. The ruins of Tanesato Castle, his base, were designated a National Historic Site in 2002 together with the ruins of Hirosaki and Horikoshi Castles as the 'Tsugaru Clan Castle Ruins.'

As the head of a minor branch family under the Nanbu, he faithfully carried out the duty of defending the Tsugaru frontier at Tanesato against the Ando clan. Little in the way of dramatic exploits is recorded of him, but the foothold he established there became the foundation upon which his descendant Tamenobu, five generations later, built an independent Tsugaru domain.

Tanesato Castle, Mitsunobu's base, is regarded as the birthplace of the Tsugaru domain, and sites associated with him — including the castle ruins and Tanesato Hachimangu shrine — remain in present-day Ajigasawa Town. His descendants, the Tsugaru clan, continued as lords of the Hirosaki domain until the Meiji Restoration.